Char is used as a fuel and is manufactured by partially pyrolyzing coal or other carbonaceous materials. Char contains sulfur which is objectionable because upon combustion, the sulfur forms sulfur dioxide, an air pollutant. Governmental air purity standards limit the use of fuels to fuels containing relatively low concentrations of sulfur. The presence in char of sulfur concentrations which exceed such limits has restricted the use of char as a fuel. The need for a relatively sulfur-free char has therefore become very important, especially in view of dwindling supplies of oil and natural gas and abundant supplies of coal.
Methods have been devised to desulfurize char but they are commercially unacceptable for removing enough of the sulfur to comply with air purity standards. One such method is to treat the char with hydrogen gas at elevated temperatures to cause a reaction between the hydrogen and the sulfur to form hydrogen sulfide gas. This method reduces the levels of sulfur present as organic sulfur and pyritic sulfur, but it increases the levels of the remaining sulfide sulfur due to reactions which convert some of the pyritic and organic sulfur to sulfide sulfur. Thus, the benefit to be gained by minimizing the pyritic and organic sulfur contents is offset by an increase in the sulfide sulfur content.
Organic sulfur refers to the sulfur which forms a part of organic molecules contained in the char. Pyritic sulfur refers to the sulfur that forms a part of iron pyrite, FeS.sub.2, found in char. Sulfide sulfur refers to the sulfur that forms a part of inorganic sulfur compounds found in char, such as, for example, FeS and CaS. Pyritic sulfur is an inorganic sulfur but is not comprehended by the term "sulfide sulfur" as used herein.
The sulfide sulfur content of char is difficult to reduce by treatment with hydrogen because the hydrogen used for such purpose usually contains trace amounts of hydrogen sulfide which inhibit the reaction between the sulfur in the char and the hydrogen gas. This inhibition to reaction with hydrogen can be reduced by increasing the temperature of reaction but this causes a corresponding increase in the amount of char that is gasified and lost by conversion to carbon dioxide.